Generally, these 6000 errors are issues with QuickBooks Online's API.
Check to make sure that your inventory items have an Income Account, Expense Account, and Cost of Goods Sold Account all selected. If you have class tracking enabled for the item, then make sure it's set to tracking per row.
Instructions for both are below.
If you have a non-inventory item, check that an Income and Expense account are both selected:
These error messages generally come with some notes, like the one below with various numbers. You can retry import on all the documents if the notes aren't clear about the error.
If your inventory item accounts are properly set, and you have class tracking enabled, then visit this guide to make sure class tracking is enabled per row.
Unexpected Internal Error. (-30004)
If you continue to get this error, please contact us for support.